Vehicles operating in outer space present a special problem in terms of being able to effectively dissipate unwanted heat generated by certain on board equipment. For example, it is necessary to get rid of the heat that is produced by electronic components and computing devices or otherwise run the risk of having such devices malfunction due to over heating. Typically, such components are mounted to a platform, panel, or other support structure which is part of the space vehicle. In addition to providing mechanical support for the component, such structure often provides the main heat sink path for cooling the component.
Spacecraft mounting platforms and panels are usually comprised of a thermally conductive facesheet bonded to one or both sides of an aluminum honeycomb core. The facesheets spread any locally imposed heat load laterally thus increasing the area from which the heat can be radiated to space. Typically, such facesheets are made of aluminum or graphite filament reinforced plastic (GFRP). Another well-known cooling technique involves the use of heat pipes, some times embedded in the support structure, to conduct heat to a remotely located radiator panel where it can be more readily rejected to space.
Such techniques and structures have proved useful and generally perform well, but not without certain disadvantages that are encountered in attaching the heat generating components and devices to the support structure. These components and devices are generally packaged in metal boxes with at least one surface designated as a baseplate from which most if not all heat dissipated within the box must be removed via thermal conduction. To remove the heat with an acceptably small temperature rise across the interface joint requires intimate thermal contact between the baseplate and the support structure over the entire baseplate "footprint" area.
Surface oxidation and microscopic irregularities and waviness (even on seemingly clean flat surfaces) create a microscopic gap over much of the area between the baseplate and the platform to which it is bolted. This microscopic gap limits heat transfer over that area to radiation coupling. Some heat also transfers across the gap via gaseous conduction in cases where atmospheric air is present. Thermal impedance imposed by the gap is often referred to as "contact resistance". Good metal to metal thermal conduction is limited to small areas directly under or very near hold-down screws where tremendous normal-direction forces can jam the peaks and valleys together. In a vacuum environment, gaseous conduction is lost but the overall heat transfer coefficient (the ratio of heat flow rate per unit of joint area to temperature difference across the joint) can still be enhanced by placing a compliant and thermally conductive medium in the gap.
The standard interface filler currently used by TRW and others for enhancing the heat transfer coefficient when mounting heat dissipating boxes to spacecraft platforms or panels is RTV (room temperature vulcanizing rubber). This material is silicone based and is initially a liquid into which a catalyst is mixed just prior to application to one or both surfaces to be joined. Excess filler material is squeezed out from the edges as the hold-down screws are tightened and is either removed in liquid form, allowed to cure in place and then cut off and removed, or simply left in place. The resulting interface joint provides good thermal contact with little or no distributed force applied to the joined members but the RTV filler creates the following problems:
Out-gassing
Even though low-volatility RTV products have been developed, a certain amount of out-gassing and potential contamination of optical or other critical surfaces still exists.
Component Removal
It is often necessary to remove an installed box or component for replacement or rework. In order to accommodate this possibility, a release agent must be applied to both surfaces prior to applying the RTV filler material to the interface joint. Even then, it can be very messy and risky to cleanup the residual RTV in preparation for reinstalling the box--especially if the box is in the form of a "slice" sandwiched between adjacent similar slice-shaped boxes.
De-lamination
When an aluminum box is mounted to a GFRP platform or panel, differential thermal expansion in the longitudinal direction can cause normal direction tensile stress in the interface filler material in the region between widely spaced hold-down screws during the hot portion of a thermal cycle. If the release agent "releases" due to this normal direction tensile stress, the mode of heat transfer across the interface joint reverts to radiation coupling resulting in a potentially catastrophic decrease in heat transfer coefficient.
One type of Configured Aluminum Foil that has been tested extensively consists of several narrow strips corrugated lengthwise and placed in a parallel pattern. Edge-to-edge spacing is made sufficiently wide to prevent interference between adjacent strips as they expand laterally during draw-down. The relatively unconstrained lateral expansion allowed by this configuration drastically reduces the normal-direction force required to flatten the interface medium. This is an important advantage for areas far removed from the hold-down screws (such as at or near mid-span) because a high distributed load tends to flex or bend the parts enough to cause severe loss of heat transfer in the center section.
A similar lengthwise flexing or bending can occur because of differential thermal expansion when aluminum boxes are mounted to GFRP (Graphite Filament Reinforced Plastic) panels. This is a potential problem for RTV filled interface joints where a release agent is used because the joint is in tension in the normal direction during high temperatures and can thus "release" when maximum heat transfer is required. Thermal cycling tests revealed that the CAF interface medium must be springy to survive. Dead-soft aluminum (1100-0) gave superior initial results but failed during the first hot-cycle after having been compressed during the cold-cycle. 5052-H191 aluminum alloy is obviously not the only springy material that could be used and may not be the best, but it does survive thermal cycling.
Electrical Grounding Impedance
It is generally required that electronic boxes be well grounded to the platform or panel to which they are mounted. In order to achieve adequate electrical grounding, gold-plated wire mesh (screen) tabs are placed in the RTV directly under each hold-down screw. In the case of an aluminum box mounted to an aluminum panel, introduction of the screen tabs leads to a slightly thicker layer of RTV than would otherwise be required. The resultant increase in weight and reduction in thermal conductance is generally acceptable. However, differential thermal expansion between an aluminum box and a GFRP panel can produce rapid degradation of electrical grounding as the few points of contact between the gold plated screen and the mating surfaces slide back and forth during thermal cycling.
Box to panel interface joints typically involve widely spaced hold-down screws. Replacing cured-in-place-RTV filler with a pre-cured sheet, gasket, or membrane of RTV or other compliant material produces a poor heat transfer coefficient for such interface joints relative to what is obtainable with cured-in-place RTV. There are at least two reasons for this difference:
Contact Resistance
Introducing any pre-cured or dry medium into the interface joint amounts to replacing the single contact resistance between the base plate and the platform with two new contact resistances--one for each side of the medium. The magnitude of the resultant thermal impedance depends on the force applied, the roughness of the mating surfaces, the softness of the medium material, and the thermal conductivity of the medium material.
Almost no normal-direction force is applied to the mating parts when using cured-in-place RTV because it is installed as a liquid and any force imposed by tightening the hold-down screws quickly relieves itself by squeezing excess RTV out the edges. Never the less, virtually 100% area contact is obtained between each of the mating surfaces and the RTV filler because each surface is wetted by RTV in the liquid state. Thus contact resistance is nil but thermal resistance through the RTV itself may be significant depending on its thermal conductivity and thickness.
Load Deflection
If a pre-cured sheet of RTV or similarly resilient material were placed between two rigid surfaces and these surfaces were then pressed together with enough force, the resulting contact resistance would probably be similar to that obtainable with cured-in-place RTV.
Real-life base plates, platforms, and panels used on spacecraft tend to flex or bow in regions between hold-down screws as a result of the distributed load associated with compressing a resilient interface gasket. The magnitude of deflection increases approximately as the cube of screw spacing. Tests have shown that for a center to center screw spacing of approximately 11 inches, deflection at the midpoint between screws often exceeds the uncompressed thickness of the gasket which of course means total loss of contact in that region.
Accordingly, there is a need for a technique for creating a reliable, durable and effective heat conductive interface that is simple to use, will endure the rigors of satellite launching and plays into the cost effectiveness of satellite construction.